During such a process there is a preheating phase for the purpose of bringing the canister and contents up to a suitable temperature before pressure can be applied for the formation of the synthetic rock structure. Typically an induction heating arrangement is utilized whereby the bellows-type canister has its walls subjected to a temperature of about 1200.degree. C. and there is a temperature gradient through the body of particulate material which is located in the canister for forming subsequently in the process the synthetic rock structure. Pressure cannot be applied until all of the material within the bellows-type canister exceeds a thresh hold temperature for the process, typically 1100.degree. C.
In the process, the bellows-type canister is subjected to hot uniaxial pressing and it is important to have a canister arrangement which compresses in a predictable and reliable manner to facilitate subsequent packing of canisters into a storage container and to avoid the risk of failure of the bellows as a reliable container. This risk would be unacceptable if the container were compressed in a manner which had uncontrolled deformation.